What
is the difference between these words: amount, quantity
and number? I mean, can I say:

A large
amount of cows were infected.

A large
quantity of cows were infected.

A large
number of cows were infected.

Are
all three possible?

Roger
Woodham replies:

amount

An
amount of something is how much of it there is that you can
measure. Amount is normally uncountable, so we CANNOT say:
'a large amount of cows were infected.'

But
we would say:

The
amount of work I got through in July was double the amount
that I did in June.

No
amount of love would heal the hatred she felt.

I
had a certain amount of respect for him: he was a good
footballer and a good ambassador for his country.

We
can also use amount as a verb, as in amount to, and
again this describes the counting or measuring of something:

When
you added everything up, his total expenditure on this project
amounted to £9,950.

I
dont think the talks in Helsinki will amount to very
much.

quantity

Similarly,
a quantity is an amount of something that you can measure
or count. We often talk about large or small quantities of
something. It is usually applied to inanimate objects so again it
is unlikely that we would say: 'a large quantity of cows
were infected'. But we would say:

There
were very small quantities of peppers on sale in the market.

There
are very large quantities of gas beneath the North Sea.

We
often contrast quantity with quality:

It
doesnt matter how many words you write: it is the quality
that is important, not the quantity.

These
toys are sold in quantity and the quality doesnt
seem to matter.

number

We
use number to describe how many, and often we do not know
exactly how many there are. This is one of the defining aspects
of a number of. Number is countable and can be applied
to both animate and inanimate items, so this is the one that fits
your sentence:

a
large number of cows were infected.

There
are a number of reasons why I cant marry you.

A
number of people were injured in the explosion.

I
had warned her not to go there any number of times, but
she wouldnt listen.

Number
in all of these instances is indefinite. Returning to your sentence,
Los, remember that if we are talking about a particular group of
cows, we would refer to them as 'a herd of cows':

The
whole herd of cows was infected.

Do
you also know the expressions: a flock of sheep or a flock of birds,
a pack of wild dogs or a pack of wolves, a pack of cards and a pack
of lies? Study the following:

In
winter, the shepherds had to move with their flocks to
the lowland pastures.

A
whole flock of seagulls followed the ferry as it set out
from Dover across the Channel.